FlixTrain refusing to reimburse for alternative arrangements

This is more of a warning but I would still appreciate some advice.

Our train from Berlin to Münster (via Hamm) was cancelled due to a mechanical problem (doors not closing properly) while we were in it, 12 mins after our supposed departure time. We were not offered any replacement, any re-routing options, any support or assistance and it was the last FlixTrain of the day. FlixTrain agent on the ground told us to make our own arrangements and contact the Help Desk for reimbursement. We found a DB train an hour later, which was x4 times more expensive than our original ticket because we bought it last at the last minute. We still took it because my partner had urgent business the following day and we were told we would be reimbursed. On the way to our destination, I submitted the ticket to the help desk, explaining the situation in great detail and providing all the information/documents necessary. A good 9 days later, I received an e-mail from them telling me that after careful review, they have decided to refund the original ticket and asking me bank account details, which I have already provided when I first contacted them.

In my reading of the EU Passenger Rights, they should reimburse for the alternative arrangement. Am I right? Did this happen to anyone before? If they don't reimburse, what are my options?

Thanks in advance.